Conventionally, boilers provided in thermal power stations and a variety of large-scale boilers employing a fuel such as petroleum, coal, or fuel gas, waste incinerators, and similar apparatuses have been equipped with a flue gas NOx removal apparatus for treating exhaust gas which apparatus includes a plurality of NOx removal catalyst layers.
The NOx removal catalyst is generally composed of a carrier (e.g., TiO2), an active component (e.g., V2O5), and a co-catalyst component (e.g., tungsten oxide or molybdenum oxide), and multi-component oxide NOx removal catalysts such as VOx—WOy—TiO2 and VOx—MoOy—TiO2 are employed.
The NOx removal catalysts typically assume the form of honeycomb, plate, etc. Honeycomb catalysts include a coated catalyst, which is fabricated by producing a honeycomb substrate and coating the substrate with a catalyst component; a kneaded catalyst, which is fabricated by kneading a substrate material with a catalyst component and molding into a honeycomb catalyst; and an impregnated catalyst, which is fabricated by impregnating a honeycomb substrate with a catalyst component. Plate-form catalyst are fabricated by coating a metallic substrate or a ceramic substrate with a catalyst component.
In any case, during use, the catalytic performance of the above catalysts is problematically deteriorated with elapse of time as a result of deposition, on the surface of the catalysts, of a substance which deteriorates the catalytic performance (hereinafter referred to as deteriorating substance) or through migration of the dissolved deteriorating substance into the catalysts.
In this connection, a variety of methods for regenerating an NOx removal catalyst have conventionally been studied.
For example, there have been studied some methods including physically removing a deteriorated portion and foreign matter so as to expose a catalytically active surface; e.g., a method including abrasion of an inner surface of a discharge gas conduit by use of an abrasive (see, for example, Patent Document 1); a method including scraping a deteriorated surface portion of an NOx removal catalyst to thereby expose a catalytically active new surface (see, for example, Patent Document 2); and a method including causing a gas accompanying microparticles to flow through a through-hole to thereby remove foreign matter (see, for example, Patent Document 3).
In addition, there have been studied catalytic performance regeneration methods through washing; e.g., a method including washing a deteriorated catalyst with an acid (pH≦5) or an alkali (pH≧8) (see, for example, Patent Document 4); a method including washing a deteriorated catalyst sequentially with water or a dilute aqueous inorganic acid solution, with a 0.1 to 5 wt. % aqueous oxalic acid solution, and with water to remove oxalic acid residing on the catalyst (see, for example, Patent Document 5); and a method including washing a deteriorated catalyst with water (50° C. to 80° C.), followed by drying (see, for example, Patent Document 6).
However, methods based on physical abrasion or a similar technique have drawbacks in that operation is cumbersome and that an NOx removal catalyst itself is cracked or broken during a regeneration process.
Generally, in the case of washing an NOx removal catalyst, an alkaline component is removed through washing with an aqueous alkaline solution, hot water, etc., and heavy metal components predominantly containing vanadium are effectively removed through washing with an aqueous oxalic acid solution. However, even though these washing-based approach are employed, washing out the deteriorating substances would be insufficient. Thus, washing-based regeneration methods employing a variety of cleaning components have been studied.
There has also been proposed an apparatus for regenerating deteriorated catalyst performance without removing the catalyst (see Patent Document 7). The approach for regenerating catalyst performance, employing an additional apparatus, incurs high operational cost, which is problematic.
As described above, a variety of regeneration methods have been studied. However, these methods have some drawbacks, and no satisfactory methods have been developed.    [Patent Document 1]
Japanese Patent Application Laid-Open (kokai) No. 1-119343 Claims and other sections)    [Patent Document 2]
Japanese Patent Application Laid-Open (kokai) No. 4-197451    [Patent Document 3]
Japanese Patent Application Laid-Open (kokai) No. 7-116523    [Patent Document 4]
Japanese Patent Application Laid-Open (kokai) No. 64-80444    [Patent Document 5]
Japanese Patent Application Laid-Open (kokai) No. 7-222924    [Patent Document 6]
Japanese Patent Application Laid-Open (kokai) No. 8-196920    [Patent Document 7]
Japanese Patent Application Laid-Open (kokai) No. 2000-325801